F26 System Wide Change: Fedora 26 C/C++ Compilation Flags Updates
= Proposed System Wide Change: Fedora 26 C/C++ Compilation Flags Updates =
https://fedoraproject.org/wiki/Changes/Fedora26CFlags
Change owner(s):
* Florian Weimer <fweimer AT redhat DOT com>
This change updates the default C/C++ compilation flags, as determined
by the redhat-rpm-config package.
== Detailed Description ==
This change covers the following modifications to the default C and
C++ compiler flags in the redhat-rpm-config package.
* Drop -mtune=atom on i686: This flag was added to improve performance
on the Intel Bonnell microarchitecture. Such CPUs are rare these days.
In addition, many users of the i686 packages download them for use
within a 64-bit x86_64 installation, and only a small part of these
Fedora installations use Atom CPUs. Current microarchitectures for
Atom CPUs are different from the original Bonnell microarchtecture and
would need different GCC flags for tuning, but many current Atom-based
devices cannot run Fedora because Fedora does not support their 32-bit
UEFI boot environment.
* Add -Werror=implicit-function-declaration: Implicit function
declarations allows a programmer to call functions without declaring
them (or including the relevant header files). The official C language
specification has not supported implicit function declarations for
almost two decades now. GCC still supports them as a GNU extension.
Implicit function declarations introduce bugs because these functions
use a different calling convention and have a fixed return type of
int. Resulting issues are pointer truncation (on 64-bit
architectures), exposure of padding bits (particular for
bool-returning functions on x86_64), and unexpected lack of hardening.
Implicit function declarations are not part of C++ (with or without
GNU extensions), and adjusting C code accordingly simplifies reuse in
C++ projects.
* Add -Werror=implicit-int: Implicit ints were removed from the C
programming language at the same time as implicit function
definitions, and were also retained as a GNU extension. Implicit ints
are usually source code bugs, and the presence of such code may
interfere with future C language directions (for example, consider how
C++ reused the auto keyword and an omitted type specifier).
The main risk from these changes are incorrect autoconf feature
checks, which often rely on technically invalid C fragments. However,
a review of existing configure scripts did not find any places where
they relied on these two problematic GNU extensions.
The technical side of this change is tracked in bug 1393492.
== Scope ==
* Proposal owners: The defaults in redhat-rpm-config need to be
changed, as described above.
* Other developers: Source code which relies on implicit function
declarations or implicit ints needs to be adjusted.
* Release engineering: This change requires a mass rebuild to become
fully effective.
* List of deliverables: No release blocking deliverables.
* Policies and guidelines: no changes proposed (change will be
implemented through redhat-rpm-config)
* Trademark approval: N/A (not needed for this Change)
--
Jan Kuřík
Platform & Fedora Program Manager
Red Hat Czech s.r.o., Purkynova 99/71, 612 45 Brno, Czech Republic